Re: The English language includes support for lists
In the first list the Oxford comma is needed to show that 'salt and vinegar' and 'cheese and onion' are individual list items - it shows you which 'and' is before the last list item. In the second list there is no confusion because the items do not contain 'and'.
In other words, use an Oxford comma when the list items include 'and'. Crisps are sold at Sainsbury's, Waitrose, and Marks and Spencer. Cheaper brands are availble from Morrison's, Aldi and Lidl.
PS 'pre' is not a preposition